Microwave ovens have come into common usage primarily for the heating and cooking of food for human consumption. Various other uses have also been developed. For instance, the microwave oven has been found to be particularly suitable for the rapid drying of various substances and determination of volatile content such as is set forth in U.S. Pat. No. 3,909,598. Further, it is known that microwave radiation greatly enhances the chemical digestion of various materials either as a result of the molecular stimulation caused by microwave radiation and/or the heating effect. Methods which use microwave radiation in acid digestion are also known.
Various developments in microwave oven technology have included the ability to program the oven to control the power input/output of the magnetron as well as the length of time that the magnetron will run. However, because most of these designs have been directed to ovens which maximize the energy input into the substance being heated such as is required for cooking, such ovens are not suitably designed for laboratory usage.
In laboratory usage, many analytical methods require the reduction or elimination of moisture in a sample, or at least, reduction to a very low level near complete dryness. This makes conventional microwave ovens unsuitable for many analytical methods. As moisture level in a sample subjected to microwave oven heating is reduced to low levels, excessive radiation energy is reflected from the oven as it can no longer be absorbed by the sample. The loss of the polar absorbing material in the sample causes the microwaves to be reflected back to the magnetron. Such reflected radiation quickly damages the magnetron.
Further, due to the characteristics of microwave radiation, it is diffucult even under ideal mechanical and size configurations based on wave length, to thoroughly disperse the radiation throughout an oven to provide uniform heating. Typical microwave ovens are best suitable for the placement of relatively large packages to be heated within the oven thereby taking up a relatively large percentage of the oven capacity. With analytical laboratory usage, normally relatively small samples are used, thus taking up a very small percentage of the oven cavity. When small samples are used, excess radiation has inadequate polar material to absorb the energy and the wave energy is thus reflected back to the source, i.e., the magnetron. Also, hot spots frequently develop in the sample which may destroy part of the sample. For most cooking purposes, localized hot spots are of little significance because they in turn aid in convection and conductive heating of the rest of the package notwithstanding the localized heating to higher temperatures in various portions of the package. Such irregular heating and the development of hot spots is particularly unsuitable for laboratory usage.